The present invention relates to electronic circuits, and more particularly, to techniques for calibrating on-chip termination impedances for transmitters and receivers to terminate transmission lines.
Signal reflection can occur on transmission lines when there is a mismatch between the characteristic impedance of the transmission line and the impedance of the transmitter and/or receiver. The reflected signal can interfere with the transmitted signal, causing distortion and degrading signal integrity.
To solve this problem, transmission lines are resistively terminated by a matching impedance to minimize or eliminate signal reflection. Input/output (I/O) pins on an integrated circuit package can be terminated by coupling external termination resistors to the appropriate I/O pins. However, many integrated circuit packages require a large number of termination resistors, because they have a large number of I/O pins. Therefore, it is becoming more common to resistively terminate transmission lines using on-chip termination (OCT) circuits to reduce the number of external components and to conserve board area.
On-chip termination circuits can be implemented with on-chip termination resistors. However, the impedance of on-chip resistors varies as the temperature of the integrated circuit changes. Also, process variations between integrated circuit die can cause on-chip resistors to have different impedances. When the impedance of an on-chip termination circuit is not equal to the characteristic impedance of the transmission line, signal reflection typically increases, causing increased signal distortion.
Because un-calibrated on-chip termination circuits can have impedances with undesirably large tolerance ranges, calibration circuits are often used to improve the accuracy of an on-chip termination (OCT) circuit. However, many prior art on-chip termination calibration circuits do not provide a high enough accuracy for some applications. Therefore, it would be desirable to provide on-chip termination calibration circuits that can provide highly accurate on-chip termination impedances.